1. Field of the Invention
The present invention relates to apparatus for testing the quality of a sheath of the polyethylene type, by detecting any defects in the sheath.
2. Description of the Background Art
Such testing is performed on all insulation sheaths. Testing is particularly strict on polyethylene-type sheaths for under-sea links. In particular, such testing must be performed on the overmolded portions of the sheaths and at the interfaces with the overmolded portions, before such a link is laid or after any repairs have been made thereto.
Conventionally, an under-sea link is made up of line cables interconnected directly in splice boxes or via repeaters. Each such repeater is clad with a casing and is provided with two access cables which are themselves connected to the line cables in splice boxes or the like commonly referred to as end boxes. The repeater casings and the splice boxes are individually protected by overmolded polyethylene sheaths co-operating with the sheaths of the cables in question to provide continuity in the sheathing of the link, so that it is waterproof and electrically insulated relative to seawater.
The sheaths of the line cables and of the repeater access cables are made of polyethylene, are extruded and are tested before the cables are used. Such testing is often performed electrically, by passing a current through the cables. When there are defects in the sheath, such testing gives rise to insulation breakdowns in the sheath. Such testing is therefore destructive.
The sheaths overmolded on the repeater casings are tested as soon as the repeaters have been equipped with their access cables, and the sheaths overmolded on the splice boxes are tested as soon as the cables in question have been interconnected.
Until now, such overmolded sheaths and their interfaces with the sheaths of the cables have been tested with X-rays, so as to detect any metallic inclusions in the sheaths or at the interfaces.
Such X-ray testing is particularly dangerous for operators, and requires highly skilled operators. It is costly and time-consuming to perform. It is also insensitive to non-metallic defects, such as for example "voids" or air bubbles, which may, like metallic inclusions, give rise to insulation breakdowns during electrical testing prior to laying the link or prior to bringing it into service.
A method of inspecting polyethylene sheaths on electrical cables is known that is based on the transparency of polyethylene to infrared radiation.
A known apparatus uses a laser emitting coherent near-infrared light to perform non-destructive testing of the quality of a polyethylene-type sheath, by detecting any defects regardless of the nature of the defects.
An object of the present invention is to provide an apparatus that is simple to implement and that makes it possible to obtain optimum resolution of defects in a cable sheath or in a sheath overmolded on an equipment casing or on a cable splice box.